1. Field of the Invention
The present invention relates to dark field illumination in the epi-illumination system, more particularly an illumination optical apparatus to be incorporated in an objective lens of an epi-illumination type microscope.
2. Description of the Prior Art
The dark field illumination is known and used for epi-illumination type microscopes in which an annular beam of light is supplied to an annular condenser member for condensing the light on the object surface. The annular beam which is emitted from the area surrounding the objective lens of the microscope, is coaxial with the optical axis of the objective lens. The annular condenser member which receives the annular beam is provided near the fore end of the objective lens. At present there is a tendency to increase the numerical aperture as well as the working distance of the objective lens. With this tendency the aperture of the objective lens is increasing more and more. Because of the large lens aperture, it is now very difficult to supply a sufficient quantity of dark field illumination light from the area surrounding the objective lens employing the conventional epidark illumination system. Besides this problem, the prior art epidark illumination system has the following disadvantages:
For those objective lenses having high magnification, the prior art system so intensively condenses the annular beam on the object surface that an image of the light source may be formed near the object surface, which often produces the problem of irregular illumination. In order to solve the problem, it has already proposed to interpose a diffusing plate in the illumination optical path. Although the diffusing plate has an effect to improve the uniformity of illumination, a substantial decrease of light intensity is caused thereby. To compensate the decreased intensity of illumination light, it is required to increase the luminance which in turn shortens the life of the light source.
Since the annular beam is heavily concentrated by the annular condenser member disposed in the vicinity of the fore end of the objective lens, there is produced aberration which also brings forth the problem of irregular illumination.